Apparatus of the type described above in the form a multi-stage screening apparatus is disclosed in Swedish Patent No. 524,527. The tubular channel of the dilution liquid header of this multi-stage apparatus has a constant cross-sectional area and the ejection nozzles of the header are identical.
A problem with this design of the prior header according to Swedish Patent No. 524,527 is that the dilution liquid flow rates in the respective ejection nozzles can vary considerably, so that the flow rates in the ejection nozzles located more downstream along the tubular channel are higher than the flow rates in the nozzles located more upstream. The varying flow rates in the different ejection nozzles result in a disadvantageous uneven distribution of the dilution liquid to the suspension.
One potential solution to this problem would be to provide uniform flow rates in the ejection nozzles of the prior header with constant cross-sectional area of the tubular channel and this could be done by designing all ejection nozzles with different sizes, so that the more upstream nozzles are wider, whereas the more downstream nozzles are narrower. A disadvantage to this solution, however, is that in practice it is very difficult to properly size the different ejection nozzles, especially when there is a great number of ejection nozzles. Besides, even if a series of ejection nozzle sizes has been correctly calculated this series is only valid for a given main dilution liquid stream supplied to the tubular channel of the header. In consequence, the distribution of dilution liquid through the ejection nozzles would not be uniform for other main liquid streams that exceed or are less than said given main liquid stream.
Another potential solution to the above problem is by providing uniform flow rates in the ejection nozzles of the prior header could be to design the tubular channel tapering in the direction of the dilution liquid stream, so that the static pressures in the dilution liquid at the entrances of the ejection nozzles are equal, which would result in equal flow rates in the ejection nozzles. However, it is complicated to design such a header, because to achieve the desired equal static pressure states at the different ejection nozzles the degree of taper of the tubular channel must be varied along the channel. In addition, the correct taper of the channel is dependent on the friction between the dilution liquid and the wall surface of the tubular channel.
One of the objects of the present invention is to improve the screening apparatus according to Swedish Patent No. 524,527, so that the header uniformly distributes dilution liquid to the suspension to be screened.
Another object of the present invention is to provide a screen basket for use in the improved screening apparatus of the present invention, and also for replacing screen baskets in existing screening apparatuses commercially operated.